With staff rosters filled, prisoners are now getting back to the normal six to eight hours out of their cells a day.

Mr Smith says inmates are supportive of the move.

"If I said to them, 'What would you ask me to do to make it better?' they said, 'Make sure each roster is staffed well.'"

But the changes have come at a cost for Serco.

Corrections pays Serco $30 million a year to run Mt Eden, but Serco must pay for the 20 Corrections staff who arrived today. It has also been pinged $500,000, mostly for serious assaults on staff, and there'll be more to come for the well-publicised incidents of the past 10 days.

At a handover this afternoon Mr Smith told staff he's there to lift the prison to the standard he expects. He told them he can do that fairly quickly, but he won't be pressed on the future of Serco's contract. That will come after the Chief Inspector of Prison's report, which won't be out in full for a couple of months.